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Introduction

The ability to generate power quickly is critical to success in competitive sport (5), and lower-body strength is a key factor in athletic development. Strength and conditioning professionals will choose both traditional and alternative methods for lower-body strength improvement when designing athletic development programs (12,23). The back squat (BS) is a bilateral exercise traditionally selected for lower back, hips, buttocks, and thigh muscle development. Consistent training with the BS has been shown to improve lower-body strength and power in both male (10) and female (11) collegiate athletes and is thought to lead to improved athletic performance (4,21).

Unilateral or single-leg training is an alternative method for developing lower-body strength and power (19,23). When included in strength program design, unilateral exercises usually serve as supplemental exercises to the BS or another bilateral exercise. Most ground-based sports require force to be produced off of a single leg when running, bounding, changing direction, and jumping, which is the rationale for the inclusion of single-leg training into the strength training program (23). The pitcher squat (PS) is a commonly used single-leg or unilateral exercise that requires standing on one leg with the opposite leg extended behind and placed onto an elevated surface (19). The lifter descends by flexing at the knee and hip until the desired depth is achieved, then returns to the starting position. Regular training with unilateral exercise (PS) has been demonstrated to be equally effective to training with bilateral exercise (BS) in previously untrained men and women (19).

Previous research has investigated trunk and lower limb muscle activity during acute bilateral dynamic exercise (BS exercise [3,7,18,22,25,32]), and acute bilateral and unilateral isometric exercise (knee extension exercise [24,26,27]) using surface electromyography (sEMG). However, studies of sEMG activity during dynamic unilateral exercise (e.g., PS) are not as frequent (16). When maximal voluntary contraction during isometric knee extension was compared between unilateral and bilateral conditions, the sum of the strengths of the unilateral contractions exceeded that of the bilateral contraction (26). This “bilateral deficit” was believed to be the result of incomplete motor unit activation (26). Vandervoort (27) reported less sEMG activity in the dominant leg during bilateral isometric exercise compared with the same leg during unilateral exercise. Schantz et al. (24) observed no difference in quadriceps femoris sEMG between bilateral and unilateral leg extension but found that the knee extension force was slightly greater in bilateral than unilateral leg extensions. Conversely, Krause et al. reported that gluteus medius sEMG activity higher during isometric unilateral than bilateral exercise during 5 weight-bearing exercises (16). Overall, there is a lack of consensus when comparing muscle activity between unilateral and bilateral exercises. Further, to our knowledge, no researchers have examined muscle activity during a unilateral multijoint, dynamic exercise, such as the PS. Regular resistance exercise training results in higher sEMG activity for the selected mode of exercise, indicating that neuromuscular adaptations are specific to the exercise selection (8). Still, little research has compared muscle activity during unilateral and bilateral dynamic exercise in resistance-trained individuals performing heavy resistance exercise (HRE).

Resistance training is widely accepted to result in muscle tissue growth and development (13,28). Although other anabolic hormones also are involved, testosterone (TES) is one of the primary anabolic hormones involved in muscle tissue growth and remodeling (28). Exercise workload (sets, repetitions, intensity), rest period, and exercise selection have been shown to have significant effects on the TES response (9,15,17), with HRE training protocols eliciting the greatest elevation in TES levels (14). This elevation of TES can have a positive effect on muscular strength development by increasing protein synthesis, lean body mass, and aiding in exercise recovery (28). Additionally, the amount of muscle mass involved must be sufficient for a change in TES concentration to occur whether the exercise is unilateral or bilateral (31). Bilateral and unilateral upper-body resistance exercise with dumbbells (bench press, bicep curl, triceps kickback, military press, bent-over row) resulted in no increase in TES levels, likely because of limited muscle mass involvement (20). Although TES is one of several primary anabolic hormones that promote adaptations to resistance exercise, total TES levels have been consistently demonstrated to be elevated with HRE (14,15,17,28). For this reason, the authors chose to examine TES in combination with sEMG during lower-body HRE training.

In summary, strength and conditioning professionals commonly select bilateral and unilateral lower-body exercises when training athletes. Yet, little research has investigated both muscular activity and total TES response during bilateral and unilateral lower-body dynamic, multijoint, HRE in resistance-trained athletes. Therefore, the purpose of this study was to determine if differences existed in muscle activation and TES and during a bilateral (BS) and a unilateral (PS) lower-body HRE.

Methods

Experimental Approach to the Problem

This study was designed to examine the total TES and muscular activity in resistance-trained, collegiate athletes performing dynamic unilateral (PS) and bilateral (BS) exercises of the lower-body. Muscle activity was recorded via sEMG. The TES was recorded via blood samples collected at baseline (preexercise) and 0, 5, 10, 15, and 30 minutes postexercise. The subjects completed 5 total testing sessions with each separated by 72–96 hours. A counterbalanced design was used for all the testing sessions. Sessions 1 and 2 were used for the determination of predicted maximal strength with the 10-repetition maximum (RM) BS and 10RM PS tests. Session 3 was for sEMG data collection where the muscle activity of the biceps femoris (BF), erector spinae (ES), gluteus medius (GM), and vastus lateralis (VL) of the right leg was recorded when the exercises were performed. The subjects performed 5 repetitions at their 10RM load of both the BS and PS exercises during this session. Sessions 4 and 5 were used to collect blood samples for subsequent TES determination during 4 sets of the 10RM load for BS and PS exercises. All testing sessions were scheduled at the same time of the day for each subject to account for diurnal variations in TES concentration (see Figure 1 for the flowchart of testing sessions). The subjects were required to refrain from use of dietary supplements for >30 days before testing.

Subjects

Ten resistance-trained male athletes (age 21.0 ± 0.8 years, height 177.3 ± 4.8 cm, mass 86.9 ± 9.6 kg), who were familiar with the BS and PS exercises and had a minimum of 1 year of formal collegiate strength and conditioning training experience, volunteered to participate in this study. The subjects were either members of the intercollegiate football team or competed as sprinters and jumpers on the intercollegiate track team. All the participants were medically cleared for Intercollegiate Athletic participation, had the risks and benefits explained to them beforehand, signed an institutionally approved consent form to participate, and completed a medical history form. The Institutional Review Board for Human Subjects approved all the study procedures. Severe musculoskeletal injuries of the lower body or spinal injuries within 6 months before the study were grounds for exclusion from the study. The subjects were instructed to refrain from leg exercise for 72 hours before each testing session. The subjects were asked to consume an identical diet for the 24 hours before each testing session and to consume nothing but water (as needed) for the 10 hours before all testing sessions. Testing sessions were conducted during the off-season training period for all the subjects.

Procedures

Ten-Repetition Maximum Strength (Sessions 1 and 2)

The subjects were tested over the first 2 testing sessions for estimated maximal strength (6) using the 10RM test for the BS and PS. A 5-minute warm-up on a cycle ergometer (model 684, Monark, Vansbro, Sweden) was completed and followed by a supervised (Certified Strength and Conditioning Specialist [CSCS]) weight warm-up before strength testing, to ensure that exercises were performed correctly. Standard weight lifting power racks (Power Lift, Jefferson, IA, USA) were used for the 10RM tests. Exercise speed was paced by a calibrated metronome (Seiko, Taipei, Taiwan) that was set to 60 b·min−1. All the subjects were required to perform the downward phase of the BS and PS exercises in 3 seconds and the upward phase in 2 seconds. During the BS exercise, an Olympic barbell and Olympic-style weights (York Barbell, York, PA, USA) were used. Safety spot bars were individually set in the power racks for each subject to ensure that he squatted to the desired depth of 90° knee flexion. During the PS exercise, the subjects held dumbbells at their sides. Standing on the right foot (stance leg), the left foot (support leg) was placed on an adjustable bench located behind the subject (19). The distance of the front leg from the bench was such that the subject was able to descend to a point where the posterior thigh was parallel to the ground and the knee of the working leg did not extend past the toes. The subjects took a timed rest of 5 minutes between each maximal effort set. Weight was increased based upon the performance of the previous attempt. If the subject completed 8 repetitions then the set was discontinued, a 5-minute rest ensued, and weight was added for another 10RM attempt. This method was continued until failure or until it was determined that the subject could no longer squat safely with proper form. During the 10RM testing for the BS, 4 subjects completed 8 repetitions and thus, required a second 10RM testing set. During the PS testing, 3 subjects required a second 10RM testing set. The average 10RM values were 120.9 ± 18.13 kg for the BS and 55.43 ± 11.69 kg for the PS, respectively.

Surface Electromyography (Session 3)

The sEMG data were collected in the third testing session using a 6-channel surface electromyography system (Model 544-Therapeutics Unlimited Inc., IA, USA). The sEMG unit had an amplification of 1 mV·V−1 with a frequency bandwidth of 20–4,000 Hz, a common mode rejection ratio of 87 dB min at 60 Hz, and an input resistance >25 MΩ. Bipolar, Ag/Ag-Cl surface electrodes with a center-to-center distance of 20 mm were used to collect the sEMG data @ 500 Hz. Before electrode application, the skin over the sites on the right leg was shaved and abraded with alcohol wipes (1,2). To ensure consistency across testing sessions, the same research assistant applied conduction gel to the surface electrodes, placed them onto the skin, and secured them with clear plastic tape. Electrodes were applied halfway between the center of the innervation zone and the further tendon of the BF, ES, GM, and VL (18,29,30). First, all completed a 5-minute warm-up on a cycle ergometer (model 684, Monark). Next, the subjects performed 5 repetitions of both the BS and PS exercises in the preset counterbalanced order. The exercise speed was paced by a calibrated metronome (Seiko) that was set to 60 b·min−1. The subjects performed the downward phase of the BS and PS exercises in 3 seconds and the upward phase in 2 seconds. Exercise intensity was equivalent to the predetermined 10RM of the subject for the specific exercise. The subjects waited for 2 seconds between each repetition. After the completion of the 5 repetitions of the first exercise, the subjects rested for a period of 5 minutes. Then, they performed 5 repetitions of the second exercise.

All sEMG data were smoothed using a 5-millisecond root mean square window. The peak muscle activation amplitude across the 5-second trial was recorded for each muscle. Thereafter, the average of the peak muscle amplitudes over the 5 trials was calculated and used for statistical analyses. Because the research design was a within-subjects design collected during a single session, we did not normalize the sEMG amplitudes. These methods have been used previously for EMG measures (1,2).

Testosterone (Sessions 4 and 5)

The fourth and fifth testing sessions were used to collect blood during the BS and PS exercises. The same trained phlebotomist performed all blood draws via an indwelling catheter inserted into the left forearm of each subject. One of the 10 subjects dropped out after a muscle strain during the fourth testing session. Therefore, blood was drawn from 9 subjects and placed into tubes without additives for subsequent TES analysis. All the samples were allowed to clot at room temperature for 15 minutes and then centrifuged for 15 minutes at 4°C at 2,000 × g. Serum was transferred into separate vials and stored at −80°C until analyzed. Testosterone was analyzed using an enzyme-linked immunoassay (Diagnostic Systems Laboratories, Webster, TX, USA), which has a sensitivity of 0.1 nmol·L−1. The samples were analyzed in duplicate in the same assay yielding an intraassay variance of <5%. These methods have been used previously for TES measures (9).

After the predetermined counterbalanced design, the subjects performed either BS or PS (10RM load) in session 4 and the remaining exercise in session 5, which followed in 72–96 hours. All the aspects of the fourth and fifth testing sessions were kept constant (e.g., time of day, warm-up, lifting cadence) with the exception of the exercise performed. During the fourth and fifth testing sessions, the HRE protocol of Kraemer et al. (14) was used to elicit a satisfactory TES response. All the subjects performed 4 sets of each exercise using their 10RMs that were previously determined in sessions 1 and 2. A rest period of 90 seconds was allowed between sets. If a subject failed to complete 10 repetitions because of fatigue on any set, the load was adjusted for the subsequent set. Load volume (load [kilograms] × repetitions completed) was calculated for each set of both exercises. Loads and load volumes for sessions 4 and 5 are included in Table 1.

Discussion

This is the first study designed to examine differences in sEMG activity and total TES response between the bilateral (BS) and unilateral (PS) dynamic HRE in resistance-trained male athletes. The overall findings supported no difference between the 2 modes of lower-body exercise.

We observed that muscle activity was similar between bilateral and unilateral exercise. Specifically, lower back (ES) and lower limb (hamstrings: BF, gluteal: GM, and quadriceps: VL) muscles responded in a similar fashion to both BS and PS, indicating that the amount of neuromuscular activity required was the same for both exercises. Our finding of similar muscle activity during both bilateral and unilateral exercise is in contrast to other observations of greater quadriceps muscle activity during unilateral than bilateral knee extension (27). Other researchers have also noted higher unilateral isometric gluteus medius activity than that in bilateral exercise (16). In agreement with our findings, Schantz et al. (24) found no difference in quadriceps muscle activity during isometric bilateral and unilateral leg extension exercises. A novel finding in this study is the findings of similar muscle activation occurred with the use of dynamic, multijoint unilateral and bilateral weight-bearing exercises instead of the commonly used single joint and isometric exercises of previous research. The EMG activity was measured on the same leg (right) for both the BS and PS exercises. When performing the PS exercise, the noninvolved (support) left leg was elevated on an adjustable bench placed behind the exercising right leg, which may have encouraged some limited muscular involvement for stabilization and balance from the left leg. When performing the BS, weight distribution may be balanced over the 2 legs, but during the PS, weight distribution is unilaterally biased, placing additional stabilization demands upon the neuromuscular system. Thus despite the lesser absolute work volumes, the subjects had muscle activation levels similar to BS, indicating that the relative loads may have been comparable between the 2 exercises.

Total TES levels increased during HRE for both BS and PS, indicating that both exercises are physiologically demanding. Specifically, TES levels increased from baseline to immediate postexercise measurements then returned to baseline within 30 minutes postexercise. Comparable TES responses have been observed previously after lower-body, HRE in both younger and older untrained men (14,17). Our finding of similar TES responses between the BS and PS resistance exercises is in agreement with previous reports of no difference in TES responses in unilateral and bilateral exercises in both the lower (31) and upper body (20).

The absolute workload volume for the PS averaged 42% of that for the BS exercise. Despite the subjects performing lesser amount of absolute work in the PS, muscle activity and total TES response values were the same across the 2 exercises. This finding is noteworthy because it indicates that effective muscle activity and TES benefits may be obtained when using unilateral HRE with less work rather than using bilateral HRE. Strength and conditioning professionals may find having their clientele train with less volume to be beneficial in the early phases of a resistance training program when the body is adjusting to the demands of the exercise.

We acknowledge some study limitations. First, in the PS, the foot of the stance (front) leg is in direct contact with the ground while the foot of the back leg is in contact with the bench. Because sEMG activity was measured only in the stance leg during the PS exercise, we can merely speculate as to load distribution on the support (left) leg. Thus, although we are cautiously confident that most work was done in the PS by the stance leg, the back leg that was on the bench may have done some work. Future researchers should examine the contributions of the back leg during PS. Second, although with unilateral exercise the expected workload volume is projected to be 50% of that performed during bilateral exercise (20), this may not always be the case when comparing unilateral vs. bilateral exercises. To standardize the relative load between the unilateral and bilateral exercises as much as possible, we had subjects perform PS and BS 10RM tests and then used these predetermined 10RM loads for the sEMG (session 3) and TES (sessions 4,5) data collection sessions. Upon secondary data analyses, we noted that during the PS exercise, subjects performed an average of 42% of their BS load-volume (load × reps completed) with values ranging across the 4 sets from 38 to 47%. The PS is a weight bearing, unilateral exercise. However, if the effect of body mass on the total load of the external resistance is taken into consideration when comparing PS to BS, and the assumption is made that all body mass was supported by the stance (right) leg, the subjects performed an average of 64% of their BS load-volume with values ranging across the 4 sets from 60 to 69%. This discrepancy in load bearing during unilateral exercises needs to be examined in future studies of unilateral vs. bilateral exercise comparisons.

Finally, the subjects in this study were trained football players and track sprinters and jumpers who were following sport-specific training regimens and involved in regular activities with definite neuromuscular demands. Thus, results may vary in untrained individuals or in athletes from other sports. Future research is needed to examine the force production and muscle activity of the support leg to clarify its role in successful PS performance. In conclusion, the PS (unilateral exercise) appears to be just as effective a multijoint, dynamic exercise for inducing muscle activity and TES changes by training as the BS (bilateral exercise). Further, because the PS is more sport specific because of its unilateral nature, this exercise should be a viable exercise alternative for strength and conditioning professionals working with ground-based sport athletes.

Practical Applications

The results of this study suggest the following in resistance-trained male athletes: (a) Strength and conditioning professionals may select lighter loads when prescribing resistance training on a single leg (unilateral exercise). Although the absolute load may be lighter, the relative intensity may be greater than or equal to that of double leg training, thereby enhancing force development off of one leg and possibly eliciting more sport-specific strength gains. (b) Muscle activity amplitudes and TES responses are similar between BS and PS exercises. Therefore, the PS may be considered an effective alternative or supplemental exercise to the BS when designing training programs for ground-based activities. (c) Changing the training stimulus with the inclusion of unilateral exercises may enhance exercise recovery and reduce overuse injury risk while still providing the same training results as bilateral resistance training exercises.

Acknowledgments

The authors wish to thank the student-athletes from the intercollegiate athletic program. They would also like to acknowledge Dr. Mary Ann Coughlin, Sarah Kelley, Karin Rood, and David Williams for their contribution to the study.